I hadn't heard of that yet. There was always a close link with the local churches when I was in primary school and secondary school (even when I was 16 I still went to the church services) and the schools would regularly observe Christian traditions. We also had the local minister who would come in and talk to the kids about various things.
The thing is though it was never in your face, never being shoved down your throat. It was just... there, really. I would rather it wasn't there but I have to admit I actually enjoyed listening to the minister when I was a little boy (I did believe in God at that time, but regardless). By my own experience, to this day most Headmasters/Headmistresses are outwardly Christian and some still go to church regularly, even as elders.
I don't know how this compares to the US, but I remember school as being quite close to the Church, with the school chaplain being a known presence. In High School, our chaplain was a radical baptist who, answering my own questions, believed in "micro evolution, not macro evolution". I just knew him though as a nice guy from Glasgow who was a bit loopy and played football with us. We saw either him or another guy with far less charisma from the Church of Scotland basically every assembly, which was once or twice a month. My favourite though was when the wank from the Church of Scotland brought over a little old Afrikaner minister with a white beard from Zimbabwe. He had really interesting stories about Zimbabwe and I liked his accent.
I am not surprised that these zealots were able to infiltrate that kind of an environment, especially if the headmistresses/teachers were of a slightly more Christian persuasion than normal.